Reaching beyond arm length in normal aging: adaptation of hand trajectory and dynamic equilibrium

The authors investigated the influence of normal aging upon equilibrium and kinematics features during a whole-body task. Eight young (23+/-1.51 years) and eight elderly (74.5+/-4.5 years) adults reached from a standing position an object placed in front of them on the ground. The authors found smaller Center of Masse (CoM) and Center of Pressure (CoP) antero-posterior displacements in elderly than in young adults. Wrist paths were curved in young but straight in elderly adults. Wrist peak velocity and duration were respectively lower and greater in elderly compared to young adults. However, Principal-Component-Analysis did not reveal differences in angle coordination between the two groups, suggesting so that modifications in equilibrium and wrist kinematics reflect an adaptation process that compensates age-related physiological changes. The authors hypothesized that equilibrium preservation in elderly contributes to wrist kinematics modifications. The authors verified this premise by placing young adults under equilibrium restrictions (reduced base of support) and observing that they reproduced the behavior of elderly adults. The authors propose that wrist kinematics is equilibrium dependent and that such a strategy is included in the motor plan of elderly adults.